Belousov–Zhabotinsky Reaction
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A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of
non-equilibrium thermodynamics Non-equilibrium thermodynamics is a branch of thermodynamics that deals with physical systems that are not in thermodynamic equilibrium but can be described in terms of macroscopic quantities (non-equilibrium state variables) that represent an ex ...
, resulting in the establishment of a nonlinear chemical oscillator. The only common element in these oscillators is the inclusion of
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...
and an acid. The reactions are important to theoretical chemistry in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior. These reactions are far from equilibrium and remain so for a significant length of time and evolve chaotically. In this sense, they provide an interesting chemical model of nonequilibrium biological phenomena; as such, mathematical models and simulations of the BZ reactions themselves are of theoretical interest, showing phenomenon as noise-induced order. An essential aspect of the BZ reaction is its so called "excitability"; under the influence of stimuli, patterns develop in what would otherwise be a perfectly quiescent medium. Some clock reactions such as Briggs–Rauscher and BZ using the tris(bipyridine)ruthenium(II) chloride as catalyst can be excited into self-organising activity through the influence of light.


History

The discovery of the phenomenon is credited to Boris Belousov. In 1951, while trying to find the non-organic analog to the Krebs cycle, he noted that in a mix of potassium bromate, cerium(IV) sulfate, malonic acid, and
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
in dilute sulfuric acid, the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated, causing the colour of the solution to oscillate between a yellow solution and a colorless solution. This is due to the cerium(IV) ions being reduced by malonic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions. Belousov made two attempts to publish his finding, but was rejected on the grounds that he could not explain his results to the satisfaction of the editors of the journals to which he submitted his results. Soviet biochemist
Simon El'evich Shnoll Simon El'evich Shnol (russian: Симон Эльевич Шноль; 21 March 1930 – 11 September 2021) was a biophysicist, and a historian of Soviet science. He was a professor at Physics Department of Moscow State University and a member ...
encouraged Belousov to continue his efforts to publish his results. In 1959 his work was finally published in a less respectable, nonreviewed journal. After Belousov's publication, Shnoll gave the project in 1961 to a graduate student,
Anatol Zhabotinsky Anatol Markovich Zhabotinsky (Анато́лий Ма́ркович Жаботи́нский) (January 17, 1938 – September 16, 2008) was a Soviet biophysicist who created a theory of the chemical clock known as Belousov–Zhabotinsky reactio ...
, who investigated the reaction sequence in detail; however, the results of these men's work were still not widely disseminated, and were not known in the West until a conference in
Prague Prague ( ; cs, Praha ; german: Prag, ; la, Praga) is the capital and largest city in the Czech Republic, and the historical capital of Bohemia. On the Vltava river, Prague is home to about 1.3 million people. The city has a temperate ...
in 1968. A number of BZ cocktails are available in the chemical literature and on the web.
Ferroin Ferroin is the chemical compound with the formula e(''o''-phen)3O4, where ''o''-phen is an abbreviation for 1,10- phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride. Redo ...
, a complex of phenanthroline and
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, is a common indicator. These reactions, if carried out in petri dishes, result in the formation first of colored spots. These spots grow into a series of expanding concentric rings or perhaps expanding spirals similar to the patterns generated by a cyclic cellular automaton. The colors disappear if the dishes are shaken, and then reappear. The waves continue until the reagents are consumed. The reaction can also be performed in a beaker using a magnetic stirrer. Andrew Adamatzky, a computer scientist in the University of the West of England, reported on liquid logic gates using the BZ reaction. The BZ reaction has also been used by Juan Pérez-Mercader and his group at
Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of high ...
to create an entirely chemical Turing machine, capable of recognizing a Chomsky type-1 language. Strikingly similar oscillatory spiral patterns appear elsewhere in nature, at very different spatial and temporal scales, for example the growth pattern of '' Dictyostelium discoideum'', a soil-dwelling amoeba colony. In the BZ reaction, the size of the interacting elements is molecular and the time scale of the reaction is minutes. In the case of the soil amoeba, the size of the elements is typical of single-celled organisms and the times involved are on the order of days to years. Investigators are also exploring the creation of a "wet computer", using self-creating "cells" and other techniques to mimic certain properties of
neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...
s.


Chemical mechanism

The mechanism for this reaction is very complex and is thought to involve around 18 different steps which have been the subject of a number of research papers. In a way similar to the Briggs–Rauscher reaction, two key processes (both of which are auto-catalytic) occur; process A generates molecular bromine, giving the red colour, and process B consumes the bromine to give bromide ions. Theoretically, the reaction resembles the ideal
Turing pattern The Turing pattern is a concept introduced by English mathematician Alan Turing in a 1952 paper titled "The Chemical Basis of Morphogenesis" which describes how patterns in nature, such as stripes and spots, can arise naturally and autonomousl ...
, a system that emerges qualitatively from solving the
reaction diffusion Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry * Chemical reaction * Nuclear reaction * Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation). Biology and ...
equations for a reaction that generates both a reaction inhibitor and a reaction promoter, of which the two diffuse across the medium at different rates. One of the most common variations on this reaction uses malonic acid (CH2(CO2H)2) as the acid and potassium bromate (KBrO3) as the source of bromine. The overall equation is: : 3 CH2(CO2H)2 + 4 → 4 Br + 9 CO2 + 6 H2O


Variants

Many variants of the reaction exist. The only key chemical is the bromate oxidizer. The catalyst ion is most often cerium, but it can be also manganese, or complexes of iron, ruthenium, cobalt, copper, chromium, silver, nickel and osmium. Many different reductants can be used. (Zhabotinsky, 1964b; Field and Burger, 1985) Many different patterns can be observed when the reaction is run in a microemulsion.


See also

* Autowave * Autowave reverberator * Briggs–Rauscher reaction * Dissipation * Excitable medium * Noise-induced order *
Patterns in nature Patterns in nature are visible regularities of form found in the natural world. These patterns recur in different contexts and can sometimes be modelled mathematically. Natural patterns include symmetries, trees, spirals, meanders, wave ...
* Reaction–diffusion * Self-oscillation * Self-organization * Stochastic Resonance *
Alan Turing Alan Mathison Turing (; 23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist. Turing was highly influential in the development of theoretical c ...
who mathematically predicted oscillating chemical reactions in the early 1950s * Brusselator * Oregonator


References


Further reading

* * *


External links


Interactive Science Experiment Showcasing the BZ Reaction (A-Level)

A Survey Article on the Mathematics of the BZ Reaction

The Scholarpedia article on the Belousov-Zhabotinsky reaction



The Belousov–Zhabotinsky Reaction


* ttps://www.youtube.com/watch?v=uWh8reiXq58 BZ reaction and explanationat The Periodic Table of Videos
The Belousov–Zhabotinski Reaction
(PDF file)

Oscillating chemical waves induced by BZ reactions can propel small objects, ''New Scientist'', 18 February 2008



A simulation of the Belousov–Zhabotinsky reaction running inside Flash Player {{DEFAULTSORT:Belousov-Zhabotinsky Reaction Name reactions Non-equilibrium thermodynamics Pattern formation Clock reactions